A new project has been pointed out to us today - phpQuery, a port of jQuery to PHP:

phpQuery is PHP-port of well known and great web2.0 JS library, which jQuery is. It's not something like JQPie, which is form of JS code generator and server-client layer.

They include a short example of adding content to a page (a new list element) that uses the DOM extension in PHP5 to make the action happen. You can check out the library as a part of their plainTemplate package.